This invention relates generally to an improved hydraulic control system for actuating a vehicle transmission. More particularly, but not by way of limitation, this invention relates to a hydraulic or electro-hydraulic control system for providing rapid smooth shifting of heavy vehicle transmissions.
Transmissions for heavy duty vehicles such as earth moving equipment frequently utilize hydraulic clutches which provide a multiplicity of speeds or ranges and also provide forward and reverse directions. The actuating and deactuation of such clutches requires that the clutch being engaged be filled rapidly and provided with sufficient pressure to fully energize the clutch.
While the foregoing have been successfully accomplished in the past, some problems have been encountered. For example, rapid filling of the clutches has frequently resulted in shift shock or jerk due to high clutch pressure at the start of clutch pressure modulation. Attempts to eliminate the jerk have required that the filling rate be lowered, and thus it takes an undesirable time to energize the clutch.
Other attempts to provide rapid fill with smooth engagement have resulted in unusually complex control apparatus in which it was sometimes possible to engage simultaneously both directional clutches or two of the range or speed clutches. This situation is, of course, extremely undesirable.
Another problem that has been encountered with such vehicles and hydraulically controlled transmissions has been the desirability of providing an emergency backup system that permits operation of the vehicle in the event of electrical failure. Most vehicles in the past have been provided with a backup system, but many have been extremely cumbersome, located poorly, or very difficult to utilize.